A new generation of optoelectronic devices such as organic photovoltaics (OPVs) and organic light-emitting transistors (OLETs) are fabricated using organic semiconductors as their active components. To be commercially relevant, these organic semiconductor-based devices should be processable in a cost-effective manner.
Bulk heterojunction (BHJ) solar cells commonly are considered the most promising OPV structures because they can be fabricated using roll-to-roll and large-scale production. BHJ solar cells include a photoactive layer disposed between an anode and a cathode, where the photoactive layer is composed of a blend film including a “donor” material and an “acceptor” material. FIG. 1 illustrates a representative BHJ organic solar cell structure.
State-of-the-art BHJ solar cells use fullerene-based compounds as the acceptor material. Typical fullerenes include C60 or C70 “bucky ball” compounds functionalized with solubilizing side chains, such as C60 [6,6]-phenyl-C61-butyric acid methyl ester (C60PCBM) or C70PCBM. The most common donor material used in BHJ solar cells is poly(3-hexylthiophene) (P3HT).
Important performance parameters for BHJ solar cells include power conversion efficiency (PCE), open circuit voltage (Voc), fill factor (FF), and short circuit current (Jsc). PCE (η) can be determined by the equation:η=Pm/(E*Ac)where Pm represents the maximum power point of the solar cell, E represents the input light irradiance (measured in W/m2) under standard test conditions, and Ac represents the surface area of the solar cell (measured in m2). Because FF is defined as the ratio of the actual maximum obtainable power, (Pm) to the theoretical (not actually obtainable) power, (Jsc*Voc), or simply FF=Pm/(Jsc*Voc), it can be seen that PCE (η) is directly correlated to each of Voc, FF, and Jsc:η=Pm/(E*Ac)=FF*Psc*Voc)/(E*Ac)
In addition, one of the fundamental limitations of solar cell efficiency is the band-gap of the donor material from which the solar cell is made. A common approach to estimate the band-gap of a π-conjugated material is to measure the optical absorption and calculate the energy at the longest wavelength onset. It is generally believed that, in order for BHJ solar cells to be commercially viable, they must achieve high Voc (>˜0.7 V), high FF (>˜65%), and an optimized Jsc. It has been shown that Jsc is optimally satisfied by absorbers having a band gap <˜1.6 eV.
It is important to point out that the Voc of a BHJ solar cell can be determined empirically by the equation:Voc=−(EHOMO(donor)−ELUMO(acceptor)+0.4)/e where EHOMO, ELUMO, and e are the Highest Occupied Molecular Orbital (HOMO) energy (or HOMO level), the Lowest Unoccupied Molecular Orbital (LUMO) energy (or LUMO level), and the electron charge, respectively. For example, the LUMO level of PCBM is about −4.3 eV. The Voc of a solar cell, therefore, also can be used to estimate the HOMO level of the donor material (given the known LUMO level of PCBM). The HOMO level of the donor material often determines its air stability. For example, state-of-the-art P3HT/PCBM BHJ solar cells typically have a Voc of about 0.6 V, suggesting that the HOMO level of P3HT is ˜5.3 eV. However, it is well known that P3HT has poor air stability. It is generally believed that a donor material having a lower HOMO value than P3HT (i.e., ˜5.4 eV or lower) will have improved oxidative stability over P3HT. In addition, as explained above, a donor material with a lower HOMO level also will likely increase the Voc of a solar cell, and in turn, its efficiency.
State-of-the-art BHJ solar cells (e.g., P3HT/PCBM-based solar cells) exhibit a PCE of ˜4-5%. However, they have several fundamental limitations. First, the band-gap of P3HT is ˜2.0 eV, and it can absorb only a small portion (30%) of the solar spectrum. It is important to develop a donor material with a band-gap of about 1.6 eV or smaller so that a larger portion of the solar spectrum can be utilized. In other words, the donor material needs to have an absorption onset at ˜800 nm or larger (the absorption onset of a material is equal to 1240 nm divided by its band-gap) to match the peak intensity region (700-800 nm) of the solar spectrum.
Accordingly, much effort has been made to develop low band-gap (<˜1.6 eV) donor materials to replace P3HT in BHJ solar cells. Although there have been a few reports of low band-gap donor-based BHJ solar cells, none of these solar cells showed much performance improvement over P3HT/PCBM-based solar cells. One problem appears to be that solar cells based on these new low band-gap donor materials still exhibit a low Voc of ˜0.6 V, indicating that the low band-gap donor materials have a HOMO level of ˜5.3 eV and thus, similar stability problems as P3HT. Also important, the BHJ solar cells based on these new donor materials typically exhibit poor fill factor (<60%).